A device for adjusting the tripping threshold of an electrical switchgear device, notably a multipole circuit breaker, having an insulating case housing a latching mechanism associated with mechanical tripping means, and an electromagnetic trip device arranged in each pole to release the latching when the current exceeds a preset tripping threshold, the trip device comprising:
a magnetic circuit having a fixed core and an excitation coil,
a plunger coupled to an operating component cooperating with the mechanical tripping means when the plunger is attracted against the fixed core,
return means biasing the plunger to a separated position in the opposite direction from the fixed core, with interposition of an air-gap whose thickness determines said tripping threshold,
means for adjusting the air-gap by means of a rotary knob for fine adjustment of the tripping threshold,
and means for selecting the angular position of an end-of-travel stop designed to limit the rotational movement of the knob to determine a maximum tripping threshold.
State-of-the-art adjustment means includes an adjustment knob coupled to the plunger of the trip device by a mechanical link. A trip device of this kind is generally used to bring about instantaneous tripping of the circuit breaker following a short-circuit current. The current flowing in the excitation coil is then sufficient to cause attraction of the plunger. The adjustment knob enables the initial position of the plunger to be modified in order to reduce or increase the air-gap for adjustment of the tripping threshold. The adjustment knob is accessible at all times, which for certain applications, for example motor startup, enabled the tripping threshold to be forced above the protection value to prevent untimely tripping during the transient startup phase. After startup, the real tripping threshold was then reset to the recommended protection value. Forcedly putting the trip device out of adjustment during the transient phase inhibits tripping of the circuit breaker, which may cause damage to the loads.